Observed and Estimated Taxonomic Diversity of Different Groups of Aquatic Organisms in the Pristine Rivers in the Biebrza National Park

•Taxonomic resources of the pristine rivers in the Biebrza National Park.•Five groups of organisms examined: benthic diatoms, phytoplankton, macrophytes, macrozoobenthos, and ichthyofauna.•Documented pool of taxa and estimated the predicted richness.•Valuable biodiversity resources, including protected plant and animal species.•High alpha diversity (within-site diversity) but relatively low gamma diversity (total diversity across all sites).